1. Field of the Invention
This invention relates to a magnetic-type absolute position encoder and, more particularly, to a magnetic-type absolute position encoder capable of detecting, as an absolute position, the rotational position, etc., of a rotary shaft of a motor or the like driving a machine tool.
2. Description of Related Art
Many encoders of optical-type have been proposed as absolute position encoders (for example, see International Application PCT/JP89/00506). An optical absolute position encoder of this kind has a rotary coding disk and a fixed coding disk, with the rotary coding disk being formed so as to obtain sinusoidal and cosinusoidal outputs the numbers of cycles per revolution of which differ from one another in each of a plurality of channels. The sinusoidal and cosinusoidal outputs in each channel enter respective A/D converters and are converted into digital data in order to successively generate information (angle information) indicative of the angular position of the rotary coding disk. Thereafter, the digital data obtained by A/D-converting the sinusoidal and cosinusoidal outputs in each channel are combined channel by channel and the result is used as address information of a ROM storing position information. A predetermined number of items of position information are interpolated in one wavelength of each channel. It is assumed that the number of channels is four, that one, 16, 256 and 4096 cycles of the sinusoidal and cosinusoidal outputs are generated per revolution in each channel, and that 2.sup.4 items of position information are interpolated in one wavelength of each channel, in which case the foregoing absolute position encoder is capable of detecting absolute position at a resolution of 360.degree./2.sup.16.
A magnetic-type absolute position encoder has not been employed in the prior art. The reason will now be explained. FIG. 6 is a diagram showing the construction of a magnetic rotation sensor (MR sensor), in which numeral 61 denotes a magnetic drum. The drum is provided with a magnetic material 62 forming a number of small poles. Numeral 63 denotes a magnetic reluctance element disposed in close proximity to the magnetic drum 61 to sense the magnetic flux of the magnetic material 62.
The magnetic drum 61 consists of a non-magnetic cylinder with a diameter of 50 mm having a magnetic paint baked onto its side face, and the magnetic paint is magnetized to have a number of poles having a pitch of amount 120 .mu.m. The magnetic reluctance element 63 comprises a magnetic reluctance element main body consisting of a permalloy thin-film pattern formed on glass, and a wiring pattern for forming a magnetic circuit. The shape and dimensions of the pattern on the main body correspond to the magnetization pattern on the drum. Thus an output signal in accordance with the amount of rotation of the drum can be obtained.
The operating principle of the magnetic rotation sensor will be described with reference to FIG. 7. The characteristic of the magnetic reluctance element 63 is such that an electrical resistance value R, which prevails when a current is passed through the pattern in the longitudinal direction thereof, decreases when a magnetic field H orthogonal to the pattern is present. Accordingly, when the pattern of the magnetic reluctance element 63 is disposed to correspond to the magnetization pattern of the drum, or in other words, when magnetic reluctance elements 63a, 63b are disposed so as to be out of phase by .lambda./4 (where .lambda. is the magnetized pattern pitch), as shown in FIG. 7(a), the resistance values of the magnetic reluctance elements 63a, 63b become smaller in alternating fashion in accordance with the movement of the magnetization pattern. In the state shown in FIG. 7(a), the resistance value of the magnetic reluctance element 63a becomes smaller and that of the magnetic reluctance element 63b becomes larger. As a result, in accordance with the rotation of the drum (the movement of the magnetized patterns), a sinusoidal signal output can be obtained from an output terminal shown in FIG. 7(b).
In actuality, as shown in FIG. 8, second magnetic reluctance elements 63', 63' are arranged at positions offset by .lambda./8 from respective ones of two first magnetic reluctance elements 63, 63 disposed so as to be out of phase by .lambda./4. These elements are connected into a bridge-like configuration, whereby the necessary sinusoidal and cosinusoidal outputs are obtained.
In the above-described magnetic rotation sensor, the magnetization pitch of the magnetic material 62 and the gap between the magnetic material and the magnetic reluctance element have a close correlation, as shown in FIG. 9. The Figure illustrates the influence of this gap upon a magnetization pitch of 120 .mu.m. More specifically, the smaller the gap with respect to the magnetization pitch, the greater the distortion in the output obtained from the sensor. The larger the gap with respect to the magnetization pitch, the smaller the output amplitude. When the gap is equal to the magnetization pitch, the sinusoidal and cosinusoidal outputs obtained have the maximum amplitude and are free of distortion.
Accordingly, in a case where an absolute position encoder is fabricated using a magnetic rotation sensor, and if it is attempted to obtain outputs of one-cycle, 16-cycle, 256-cycle and 4096-cycle sinusoidal and cosinusoidal signals employed in the abovementioned proposed optical absolute position encoder, the difference in the magnetization pitch of each channel will be too large and the gap possessed by the magnetic reluctance element 63 (or 63') must be provided with steps. Thus, problems arise in terms of construction.
In addition, an arrangement in which sinusoidal and cosinusoidal signals having a low number of cycles (e.g., one cycle) per revolution are generated magnetically is impossible to realize. Further a magnetic-type absolute position encoder having a simple structure cannot be obtained.
Accordingly, an object of the present invention is to provide a magnetic-type absolute position encoder in which the gap of the magnetic reluctance element of each channel can be made uniform, sinusoidal and cosinusoidal signals having a low number of cycles can be generated, and the absolute position of a moving body can be detected.